In old conventional Prior Art for cooking of chemical cellulose pulp with continuous digesters it was common to use a pre-treatment arrangement with a chip bin, steaming vessel and an impregnating chip chute, before the cooking process is established in the digester. Steaming has been carried out in one or several steps in the chip bin, prior to the subsequent formation of a slurry of the chips in an impregnation fluid or a transport fluid. The steaming has been considered to be absolutely necessary in order to be certain of expelling the air, such that the impregnation fluid can fully penetrate the chips, and such that air is not drawn into the system.
Attempts have been made to integrate the chip bin with the impregnation vessel in order to obtain in this manner a simpler system.
Already in U.S. Pat. No. 2,803,540, a system was revealed in which the chips from a chip bin were fed to a vessel in which a combined steaming and impregnation was achieved. In this vessel, the chips were steamed at the upper part of the vessel and impregnation fluid at the same temperature was added at various levels in the vessel using distributing annular manifolds/headers located outside of the vessel wall, and having nozzles penetrating the wall. These principles were also applied in a process known as “Mumin cooking”, which is described in “Continuous Pulping Processes”, Technical Association of the Pulp and Paper Industry, 1970, Sven Rydholm, page 144. In this process, unsteamed chips were passed to a combined impregnation vessel, where steaming was obtained in the upper part, and to which impregnation fluid was added at a point in the upper part of the vessel during forced circulation. The impregnation fluid was in this case carried exclusively in the same direction of flow as the chips.
A similar system with a low pressure first common steaming and impregnation vessel is shown in U.S. Pat. No. 3,532,594, which also was put into operation for example at the Skoghall mill in Sweden. Here was heated impregnation liquid added via a central pipe to the chip volume, but also showing an additional central pipe for steam supply. This system was later abandoned due to various reasons such as run ability problems, capacity problems in subsequent feeding system and far too high reject and shive content in the blown pulp.
A system is shown in U.S. Pat. No. 5,635,025 in which the chips are fed without prior steaming to a vessel in the form of a combined chip bin, impregnation vessel and chip chute. Steaming of the chips takes place here, the chips lying above the fluid level, and a simple addition of impregnation fluid takes place trough the vessel wall below the liquid level.
A further such system is revealed in U.S. Pat. No. 6,280,567, in which the chips are fed without prior steaming to an atmospheric impregnation vessel in which the chips are heated by the addition of warm black liquor that maintains a temperature around 130-140 C. The added impregnation liquid is added via nozzles in a manner similar to what is shown in U.S. Pat. No. 2,803,540, i.e. using supply nozzles penetrating the wall of the vessel.
An alternative system is revealed by SE 523,850 in which pressurized black liquor is added to the upper part of the steaming vessel, whereby the black liquor after being subjected to a pressure reduction releases steam for the steaming process. The addition of the hot black liquor is made by a stationary horizontal supply pipe penetrating the chip bed, and having a multitude of holes over the length of the pipe. Even if the distribution of hot black liquor is made over a larger (but not over the complete cross-sectional) area this solution is not advisable since the horizontal pipe may hinder the chip plug movement. The prior art has mostly used either a stationary central pipe or stationary annular distribution nozzles located in the wall of the vessel for the added impregnation liquid to the combined steaming and impregnation vessel.
During the last few years the design capacity of new digester systems has been significantly increased, from typically 500-2000 ADt/24 h to production rates over 5000 ADt/24 h. As the design production rate increases the combined chip steaming and impregnation vessel diameter increases as well.
The technique with a common treatment vessel for steaming and impregnation at substantially atmospheric conditions is marketed by Metso Paper under the name of IMPBIN™. Several improvements of the concept have been patented in;                SE 518.738 (=U.S. Pat. No. 7,381,302), with impregnation liquids added at successively increasing temperature at stationary positions in IMPBIN with higher static head;        SE 528.448 (=EP1818445), with liquor circulations of IMPBIN separated from those in digester;        SE 530.725 (=EP2065513), with cooling showers in top of IMPBIN for knocking down blow trough of malodorous gases.        
Another disadvantage with known solutions is that this common steaming and impregnation vessel should optimize both functions, and it is a conflicting interest, as both steaming effect and impregnation time needs may vary depend on differing operating conditions and type of cellulose material fed to system. If there is a need to extend the impregnation time in the impregnation liquid it may not be possible to increase the liquid level, as such an increase may reduce the time in the steaming zone, or may reduce the amount of steam generated by flashing hot liquors. Flashing off steam beneath the liquid level creates a boiling effect that disrupts the formation of a well packed chip pile in the center of the vessel, which may cause a channeling effect inside the vessel hampering a uniform treatment of the cellulose material inside the vessel.
The above mentioned disadvantages with possibilities of extending the impregnation zone for wood material that is difficult to impregnate, or if other process conditions call for an increased liquid level in the impregnation zone, while still maintaining an efficient steaming effect, has become apparent after a large number of implementations of IMPBIN, especially those being fed with cellulose material of differing qualities during operation.
The principle object of the invention is to obtain an improved and more flexible arrangement for the addition of treatment liquids to chips during the manufacture of chemical pulp in a continuous process using a down flow vessel where chips are descending down the vessel in a plug flow, which arrangement does not demonstrate the disadvantages that are associated with other known solutions as described above.
A specific objective is to enable changes of the liquid level while adding steam, from a supply of steam and/or from flashing hot liquor, which addition of steam is controlled to take place very closely above the liquid level such that an optimum steaming effect is at hand independently of the established liquid level.
Another specific objective is to be able to change the retention time in the impregnation zone when changes in wood quality occur. These changes in wood quality could depend upon                type of wood (hardwood/softwood/annual plants) or mixture thereof;        changes from winter to summer seasons (altered amount of ice brought with chips and temperature of chips);        Wood chip quality (size of chips, pin chips/ordinary chips);        Moisture content of chips;        Chip bulk density (hardwood VS softwood but also changes within each type).        
Yet another specific objective is to be able to reduce the torque demand on mechanical feed means in bottom of the down flow vessel, typically a bottom scraper. When the chip quality occasionally is outside of specifications, for example a high content of fine wood particles (sawdust or pin chips fractions are high), then the compaction of the chip column in the vessel might increase to such extent that the working load on the bottom scraper becomes excessively high. This may cause operational disturbances and production losses. To counter-act too high chip column compaction it is common practice to reduce the difference between the chip and liquor level in the vessel by increasing the liquor level so that the downward thrust from the chips being located above the liquid level is reduced. It is important that during such control should not the point of introduction of steam and/or hot liquor be drenched by the rising liquid level, which would reduce an effective generation of steam.
The invention can advantageously be used when cooking hard wood or softwood chips, bagasse, bamboo or other annual plants.
The characteristics of the invention are defined by the independent claims, and optional embodiments are defined in dependent claims in order of dependency of preceding claims. The invention is also disclosed in a preferred embodiment, but any specific feature of this embodiment could as such be included in the invention optionally, if not specifically defined as a necessary feature for the argued effect.